(NaturalNews) An increasing number of U.S. girls are now reaching puberty as young as the age of seven, according to a study conducted by researchers from Cincinnati's Children's Hospital and published in the journal Pediatrics.

Researchers examined more than 1,200 girls between the ages of six and eight who were living in Cincinnati, San Francisco or East Harlem, N.Y. twice between 2004 and 2006. Each examination was conducted by a different female pediatrician, who felt the girls for the development of breast tissue.

The researchers found that 10.4 percent of white girls, 15 percent of Hispanic girls and 23.4 percent of black girls had developed breast tissue by the age of seven. In contrast, one study 10 years ago found that only 5 percent of white girls and 15.4 percent of black girls had developed breast tissue by that age.

The prior study did not include Hispanic girls.

The new study further found that 18 percent of white girls, 31 percent of Hispanic girls and 43 percent of black girls had breast development by age eight. The prior study found breast tissue in only 10.5 percent of white girls and 36.6 percent of black girls at that age.

Early onset of puberty can expose girls to both physical and social problems. It has been associated with a higher lifetime risk of breast cancer, and can expose emotionally immature young girls to sexual advances from older boys and men.

Researchers believe that rising rates of obesity and increasing exposure to hormone-disrupting chemicals are the two major causes of early puberty. In the book Growing Up Green, Deirdre Imus quotes a New York Times article as saying, "some physicians worry that children are at higher risk of early puberty as a result of the increasing prevalence of certain drugs, cosmetics and environmental contaminants, called 'endocrine disruptors,' that can cause breast growth, pubic hair development and other symptoms of puberty."

The current study found that early onset of puberty was correlated with a higher body mass index (a measure of obesity). The researchers are now conducting followup urine tests to look for the effects of toxic chemicals.

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